1. Field of the Invention
This invention relates to a scanning image formation optical system, an optical scanner, and an image forming apparatus.
2. Description of the Background
An optical scanner is widely known in connection with an image forming apparatus such as, for example, a digital copying machine, an optical printer, and an optical plotter. The optical scanner is generally configured such that a coupled luminous flux from a light source is deflected by an optical deflector such as a polygon mirror, the deflected luminous flux is condensed toward a scanned surface by using a scanning image formation optical system so as to form an optical spot on the scanned surface, and optical scanning is performed on the scanned surface by the optical spot.
When using an optical deflector of a type in which a deflecting reflective surface rotates around a rotary axis parallel to the deflecting reflective surface, such as a polygon mirror, two methods, described below, are well known for causing a coupled luminous flux transmitted from the light source to be incident on the optical deflector.
The first method is to cause the luminous flux to be incident on a plane perpendicular to the rotary axis in a direction almost parallel to the plane (referred to as the “normal incidence method”), and the second method is to cause the luminous flux to be incident on the plane perpendicular to the rotary axis in a direction diagonal to the plane (referred to as the “diagonal incidence method”).
The diagonal incidence method has the following advantages and disadvantages in comparison with the normal incidence method. More specifically, when using a polygon mirror as an optical deflector, for example, it is hard to cause a luminous flux from the light source to be directed toward the rotary axis of the polygon mirror in the normal incidence method. It is not impossible to cause the luminous flux to be so directed, but if an attempt is made to secure a required angle of deflection when it is directed toward the rotary axis, each deflecting reflective surface becomes extremely large, and thereby the polygon mirror cannot be downsized. In addition, a large sag may be generated, and the generated sag is asymmetrical relative to the image height.
A large polygon mirror requires high energy for its high-speed rotation and makes a loud whizzing sound when rotated at a high speed, and therefore it is inevitable to have to increase the size of a sound insulating device in the normal incidence method.
Contrary to this, in the diagonal incidence method, it is possible to cause a luminous flux from the light source to be directed toward the rotary axis of the polygon mirror, by which the polygon mirror can be downsized in its diameter and only a little whizzing sound is made when the polygon mirror is rotated at a high speed. Therefore, the method is suitable for a high-speed optical scanner. Because the polygon mirror can be downsized in its diameter, just a little sag may be generated, and the generated sag can be symmetrical with respect to the image height, thereby facilitating correction of the sag.
On the other hand, however, the diagonal incidence method has a problem in that a significant scanning line curvature is present.
As a method of correcting the significant scanning line curve inherent in the diagonal incidence method, a method has been proposed to add a lens having a lens surface in which an inherent tilt of the lens surface in a sub-scanning cross-section is shifted in the main scanning direction, so as to correct a scanning line curve, to a scanning image formation optical system (Japanese Patent Laid-open publication No. 11-14932). Also, a method has been proposed to add a correction reflective surface having a reflective surface in which an inherent tilt of the reflective surface in a sub-scanning cross-section is shifted in the main scanning direction, so as to correct a scanning line curve, to a scanning image formation optical system (Japanese Patent Laid-open Publication No. 11-38348).
Another problem of the diagonal incidence method is in that relatively significant deterioration is easily caused in the wavefront aberration of a scanning image forming optical system by a beam skew at each peripheral image height, i.e., in the vicinity of both ends of a scanning line. An occurrence of wavefront aberration increases a spot diameter of an optical spot at the peripheral image height. Unless the above-described problem is resolved, high-density optical scanning, which has been demanded in recent years, cannot be achieved. In the optical scanner described in the above publications, a large scanning line curve inherent in the diagonal incidence method is corrected very favorably, but correction of deteriorated wavefront aberration is not enough.
As an optical scanner capable of favorably correcting the above-described deterioration of scanning line curvature and wavefront aberration, which is a problem of the diagonal incidence method, an optical scanner has been proposed in which a plurality of rotary asymmetrical lenses are added to a scanning image formation optical system and in which a generating line joining vertices of generated lines of the lens surfaces of these rotary asymmetrical lenses is curved in the sub-scanning direction (Japanese Patent Laid-open Publication No. 10-73778).
The lens having a lens surface in which a generating line joining vertices of generated lines is curved in the sub-scanning direction, as described above, has a curved generating line. Therefore, the width of the lens in the sub-scanning direction has to be increased. Particularly, when the lens surface has a relatively large curvature, an amount of curvature of the generating line for correcting the scanning line curve is increased, thereby requiring the lens width to be considerably increased.